A fuel cell normally has a pair of electrodes comprising an anode and a cathode sandwiching a membrane electrolyte. Fuel gas containing hydrogen is supplied to the anode. A gaseous oxidant containing oxygen is supplied to the cathode. Electrochemical reactions occurring on surface of the membrane electrolyte produce a voltage.
The reaction shown in Equation (1) occurs on the anode.H2→2H++2e−  (1)The reaction shown in Equation (2) occurs on the cathode.2H++2e−+(½)O2→H2O  (2)
Hydrogen gas or a fuel gas containing hydrogen supplied by a hydrogen storage device is used as the fuel gas. The fuel gas is a hydrogen-rich gas resulting from the reforming operation applied to natural gas, methanol or gasoline for example. Air may be used as the gaseous oxidant.
In the prior-art example, two gas diffusion electrodes, each comprising a catalyst layer and a gas diffusion layer, are provided as an anode and cathode to sandwich the membrane electrolyte, forming a “membrane-electrode-assembly” (MEA). A current collector is provided to function as a channel-forming member on the outer side of the gas diffusion electrodes. A fuel cell stack is formed by laminating unit cells each containing a MEA, a pair of current collectors sandwiching the MEA, and a separator for separating adjacent unit cells. Normally the current collector(s) and the separator are integrated to form a plate-shaped member called a bi-polar plate.
The bi-polar plate serves as an electrically conductive member for creating an electrical connection between unit cells. It also functions as a partition for separating gas supplied respectively to the anode and cathode and has the function of maintaining the gas flow channel. Since it is necessary to maintain the homogeneity of gas flow, the bi-polar plate must be manufactured using highly accurate processing operations.
A bi-polar plate is sometimes porous from the point of view of water management. Water management means the recovery of produced water and humidification in the gas in the fuel cell. The bi-polar plate is usually moisturized by pure water flowing in a pure water channel formed on/in the bi-polar plate. In other words, the water flows into the porous sections of the bi-polar plate. When the moisturized bi-polar plate comes into contact with dry gas in the gas channel, water vaporizes from the porous sections if the vapor pressure of the supplied gas has not reached a saturation vapor pressure. Furthermore water produced as a result of Equation (1) and (2) is absorbed into the porous bi-polar plate as a result of setting the pressure in the gas channel to be higher than the pressure in the pure water channel. In this manner, excess water is removed from the gas channel and can be prevented from blocking the gas channel.
However since a bi-polar plate used for the purpose of water management as described above presents manufacturing difficulties due to its low mechanical strength.
Tokkai Hei 11-283636 published by the Japanese Patent Office in 1999 discloses a bi-polar plate formed by laminating a plate provided with linear slits and a flat plate. Tokkai Hei 7-45294 published by the Japanese Patent Office in 1995 discloses a plurality of thin pieces comprising the current collector which are disposed at intervals on a flat plate acting as the separator. The thin pieces define the gas channel.